1. Field of the Invention
The present invention relates to a method for supplying power to induction cooking zones of an induction cooking hob with power converters, each of such power converters feeding an inductor.
2. Description of the Related Art
An induction cooking system comprises two main components; an AC/AC power converter (usually of the resonant type) that transforms a mains line voltage (ex. 230V, 50 Hz in many EU countries) into a high frequency AC voltage (usually in the 20-50 kHz range) and an inductor that, when a cooking vessel is placed on it, induces a high frequency magnetic field into the cooking vessel bottom that, by Joule effect caused by induced eddy current, heats up. It is desirable that the power delivered to the cooking vessel can be adjusted, according to the recipe chosen by the user, from a minimum to a maximum power, and such feature can be obtained by adjusting some working parameters of the AC/AC converter, such as the operating frequency of the output signal and/or the operating voltage of the output signal.
When an induction cooking system comprises more than one inductor, some electric or magnetic coupling may exist between the AC/AC converters and/or the inductors, or a limitation on the sum of the power delivered by the inductors may exist because of limited rating of the mains line power. The electric or magnetic couplings result in generation of audible noise when two coupled converters or inductors are operated at different frequencies (whose difference lies in the audible range) and cause excessive disturbances on the mains line that can exceed the standard compliance limitation. Furthermore the mains line rating limitation on the maximum available power requires that a common control prevents the total power delivered by the converters connected to a mains line from exceeding the prescribed limit.
To avoid audible disturbances when operating two coupled induction cooking systems (each having AC/AC inverter plus inductor) both systems may be operated at the same frequency or at frequencies whose difference lies outside the audible range. The operation at different frequencies can result in increased mains line disturbance level, so that it is preferable to avoid this condition. In order to allow the required flexibility in the power setting and adjustment, the operating voltage of the AC/AC converter should be used as control parameter.
Changing the output voltage is difficult to implement cost effectively for resonant converters normally used in induction cooking systems.
For half bridge series resonant converters, among the possible ways to change and therefore adjust the output voltage, is to operate on the power switches activation duty cycle. Deviating from the standard operating condition of the switches control (duty cycle=50%) can result in loss of soft switching working condition on the power switches, and severe switching loss increase can lead to overheating and failure of the devices. The method of changing the output voltage should be used only for “small” changes (approximately for a power regulation in the range 2:1, which allows to keep the soft switching condition) but the required flexibility for commercial induction cooking systems is to have a power ratio as high as 100:1. Other methods of changing the output voltage (for example using silicon-controlled rectifier SCR on the rectifying bridge to reduce the mains voltage rms value, or introducing a Boost or Buck regulator ahead of the half bridge circuit), require additional costs that are not economically attractive for the market. A technical solution of this kind is disclosed by EP-A-1895814.
Audible noise generation can be avoided as described in WO 2005/043737 where the operation of two coupled induction systems is allowed when the frequency difference lies outside the audible frequency range (˜20 Hz-20 kHz). By combining this feature with the voltage change, a higher flexibility in the operation can be obtained, but higher disturbance level is generated on the mains line.
The power can be limited with an ON/OFF operation of an induction system. For example, to get 500 W out of a converter, the latter can be operated at 1000 W for half of the operating time. This method becomes effective when the control cycle time is much smaller than the thermal time constant of the cooking vessel, so that the average power is delivered to the food being cooked without the user perceiving the power modulation.
This method described above can be used alone to control the delivered power only with special care, since it can involve big power steps, and consequently high flicker values that can cause the product to fail the standard IEC relevant test. Therefore, the power step must be kept low or the cycle time must be made high enough to limit the flicker value, but a limit exists such that the cycle time should be much smaller than the cooking vessel thermal time constant, otherwise the customer will strongly perceive the ON/OFF modulation in the cooking process.
A similar control method for controlling two inductors is described in EP-A-1951003, and it solves the problem for a cooking system made of two inductors coupled by the mains, as shown in the attached FIG. 2. The solution disclosed solves only one of the coupling problems at a time, but it is not able to solve the whole problem of several power converters and inductors, because it does not create enough freedom in the system to match the user setting and the system constraints.